= Solution
\b[The most acidic sites are: acetic-acid O–H, $\mathrm pK_a\approx4.8$; phenol O–H, $\approx10$; ethanal alpha-CH$_3$, $\approx17$–$20$; propyne terminal ≡C–H, $\approx25$.] These are ordinary approximate organic-chemistry estimates; exact acidities depend on solvent and reference convention.
<Acetic acid> gives a carboxylate in which negative charge is delocalized over two oxygens through equivalent <resonance structures>. <Phenol> gives a phenoxide with delocalization into the aromatic system, but its important contributors are less equivalent and do not stabilize the charge as effectively as a carboxylate.
For ethanal, removal of a methyl hydrogen gives the <enolate> $\mathrm{^-CH_2-CH=O\leftrightarrow CH_2=CH-O^-}$. The aldehydic hydrogen is not the preferred acidic site: removing it does not give this alpha-carbon/oxygen delocalization. For <propyne>, loss of the terminal hydrogen gives an acetylide with a lone pair in an $sp$ orbital. Its greater s character stabilizes negative charge relative to an ordinary $sp^2$ or $sp^3$ carbanion; the methyl hydrogens are much less acidic.
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